The present invention is generally directed to vehicle suspension systems, and in particular to vehicle suspension systems incorporating improved passive ride control.
The desire for improved ride control in motor vehicles has lead to the development of xe2x80x9cactivexe2x80x9d vehicle suspension systems. Such systems typically use sensors to sense the various ride characteristics of the vehicle, the sensors providing signals to an Electronic Control Unit (ECU). The sensors sense any excessive roll, pitch, four wheel bounce and warp motions of the vehicle and its wheels, and the ECU seeks to actively compensate for this motion by controlling the supply of high pressure fluid from a fluid pump to different actuators acting within the vehicle suspension system, or by controlling the return of high pressure fluid from the actuators to a fluid reservoir. (The warp mode of a suspension system, also known as cross axle articulation, is defined as when one pair of diagonally spaced wheels together move in the opposite vertical direction to the other pair of diagonally spaced wheels with respect to the vehicle body). Active suspension systems which attempt to control all the above-noted ride characteristics are very expensive and complicated and have therefore not proven to be commercially viable. Simpler active systems which only seek to actively control excessive roll motions of the vehicle have therefore also been developed. Similarly, adaptive damping systems are becoming popular as they can be used to influence vehicle motions such as roll, pitch and whole body bounce by changing the damping rates at each wheel without the need for a pump.
All the known active suspension systems however have a number of problems which have prevented commercial acceptance of such systems except in luxury vehicles. The number of components required for such systems have lead to packaging difficulties, with the limited space available for such systems under existing motor vehicles. The complexity of active suspension systems and the high stresses applied to certain components of the system lead to ongoing reliability issues. Furthermore, active systems typically require a large number of components, some of which are specially produced components that can handle high mechanical stresses leading to high manufacturing costs. Also, expensive high pressure and high speed components are typically used in such systems, resulting in relatively higher manufacturing and running costs for active systems when compared with conventional suspension systems. Another disadvantage of active systems is the poor response times generally associated with production feasible versions of such systems. Valves are generally used to control the fluid flow in the system. There is always a certain delay before a valve can be actuated to allow or prevent fluid flow. This delay, together with other delays caused by inadequately defined algorithms controlling the system, can lead to unacceptably poor response times for the active suspension system. Active roll control systems typically respond too slowly when undergoing a quick slalom test for example, the control system being unable to provide adequate control under large changes of inertia.
The Applicant has developed a number of different vehicle system systems which seek to avoid at least some of the problems associated with active suspension systems while providing substantial improvements in the ride of a vehicle. These systems are xe2x80x9cpassivexe2x80x9d and do not require sensors, ECUs or fluid pumps to operate. Such systems are described in Australian Patents 670034, 694762, 671592 and 699388 and International Application No. PCT/AU97/00870, details of which are incorporated herein by reference. These systems do however generally rely on components adapted to handle high pressure fluid.
Adaptive damping systems have been developed specifically to improve the damping function of a vehicle suspension system. These damping systems only require relatively low pressure components when compared with those required in the previously described systems, but provide substantially no roll stiffness. They generally have electrically variable or switchable orifices and preloads which are controlled to provide more appropriate damper forces in a range of predefined conditions to avoid the compromises of a single setting to suit all conditions.
In U.S. Pat. Nos. 5,486,018 and 6,684,496 (Yamaha), there are described interconnected damper systems where the top chamber of at least one pair of laterally or longitudinally adjacent dampers, commonly known as {dot over (O)}shock absorbers"" are connected by a conduit. A number of arrangements are disclosed, providing a range of damping effects. However, none of the arrangements are designed to provide a roll stiffness for the suspension.
In U.S. Pat. No. 4,606,551 (Alfa), there is described an arrangement having dampers, each having an upper and lower chamber. At least one pair of laterally or longitudinally adjacent dampers are connected by conduits respectively connecting the upper chamber of one damper with the lower chamber of the other chamber. A number of damper valves are provided in the connecting conduits to provide various damping effects. No electronic control is required, nor can the arrangement provide a roll stiffness for the suspension.
Although each of the above described adaptive and interconnected damping systems provide an improved damping function over conventional damper arrangements, they do not provide any or only provide minimal control of other ride characteristics of the vehicle. For example, none of the above adaptive or interconnected damping systems provide roll support for the vehicle as they do not have any roll stiffness to enable a degree of roll control for the vehicle, only roll damping. These systems can therefore not be used to provide roll control for the vehicle.
It is therefore an object of the present invention to provide a damping and roll control system which achieves improved ride control for the vehicle while avoiding at least one of the problems associated with prior art systems.
With this in mind, the present invention provides a damping and roll control system for a vehicle suspension system the vehicle having at least one pair of laterally spaced front wheel assemblies and at least one pair of laterally spaced rear wheel assemblies, each wheel assembly including a wheel and a wheel mounting locating the wheel to permit movement of the wheel in a generally vertical direction relative to a body of the vehicle, and vehicle support means for providing at least substantially a major portion of the support for the vehicle; the roll control system including:
wheel cylinders respectively locatable between each wheel mounting and the body of the vehicle, each wheel cylinder including an inner volume separated into first and second chambers by a piston supported within the wheel cylinder; first and second fluid circuits respectively providing fluid communication between the wheel cylinders by fluid conduits, each said fluid circuit providing fluid communication between the first chambers of the wheel cylinders on one side of the vehicle and the second chambers of the wheel cylinders on the opposite side of the vehicle to thereby provide roll support decoupled from the warp mode of the vehicle suspension system by providing a roll stiffness about a level roll attitude whilst simultaneously providing substantially zero warp stiffness;
each fluid circuit including one or more fluid accumulators for providing roll resilience;
damper means for controlling the rate of fluid flow out of or into at least one chamber of each wheel cylinder, and
selection means for selectively providing fluid communication between the first and second fluid circuits;
the damping and roll control system thereby providing substantially all of the damping of the vehicle suspension system.
The vehicle support means may in certain embodiments of the present invention provide at least substantially all of the support for the vehicle.
The damping and roll control system therefore provides damping for the vehicle suspension and provides a roll stiffness without introducing a corresponding warp stiffness.
Each fluid circuit may in one preferred embodiment include a first fluid conduit providing fluid communication between the first chambers of the wheel cylinders on one side of the vehicle; and a second fluid conduit providing fluid communication between the second chambers of the wheel cylinders on the opposite side of the vehicle; the first and second fluid conduits being in fluid communication.
According to another preferred embodiment, each fluid circuit may include first and second diagonal fluid conduits, each respectively providing fluid communication between the first chamber of one wheel cylinder on one side of the vehicle and the second chamber of the diagonally opposite wheel cylinder on the other side of the vehicle; the first diagonal fluid conduit between one pair of diagonally opposite wheel cylinders being in fluid communication with the second diagonal fluid conduit between the other pair of diagonally opposite wheel cylinders.
According to yet another preferred embodiment, each fluid circuit may include a front fluid conduit providing fluid communication between the wheel cylinders of the front wheel assemblies, and a rear fluid conduit providing fluid communication between the wheel cylinders of the rear wheel assemblies, with the front and rear conduits respectively providing fluid communication between the first chamber of the wheel cylinder at one side of the vehicle with the second chamber of the wheel cylinder at the opposite side of the vehicle, the front and rear conduits being in fluid communication.
It is to be appreciated that other connection arrangements are also envisaged. It is also to be appreciated that the same principles may be applied to vehicles with more than four wheels. For example, to apply the system to a six wheeled vehicle, the additional left hand wheel cylinder will have its first chamber connected to the conduit connecting the first chambers of the other two left hand wheel cylinders, and its second charmer connected to the conduit connecting the second chambers of the other two left hand wheel cylinders. The connection of the other cylinder to the right hand side of the vehicle similarly communicates first chambers together and second chambers together.
The damper means may be located at or in the wheel cylinders, in the conduits, and/or in a manifold block. The manifold block may be centrally located in the vehicle and may provide the required fluid communication between the first and second conduits to form the first and second fluid circuits, The damper means may be a bi-directional valve (ie. provide controlled flow restriction in both directions), in which case each wheel cylinder requires only one damper valve for one of the first or second chambers. In this case, the associated chamber may try to suck a vacuum if the damper valve is not supplying fluid at the same rate at it is being demanded. This can lead to aeration of the fluid and potential loss of ride control by the system. To avoid this effect, a single direction damper valve may be used to ensure that the wheel cylinder chambers only act through a damper valve when expelling fluid, thereby preventing fluid aeration in the cylinder chambers. Alternatively, the single direction damper valve may be used in parallel with a non-return valve. Alterative, to provide large damping forces with reliable, compact damper valve means, a bi-directional damper means may be provided for each of the first and second chambers of at least one pair of laterally spaced wheel cylinders.
Each said fluid circuit includes at least a first fluid accumulator to allow for changes in the fluid volume of each circuit to thereby provide roll resilience. Also, if a wheel cylinder with differing effective piston areas between the first and second chambers is used (for example a piston having a rod extending from one side only, as in a conventional damper cylinder assembly), the accumulator needs to be able to accommodate the rod volume changes within the system during bounce motions of the suspension. In this case, in roll, the accumulator absorbs a much greater change of fluid volume per unit displacement of the wheel cylinders than it absorbs in bounce as both the effective areas of a first chamber side and a second chamber side are working to displace fluid into the accumulator giving a correspondingly higher stiffness for roll motions of the roll control system than for bounce motions.
Each fluid circuit may include at least one second fluid accumulator to provide increased roll resilience. Between each second accumulator and the respective fluid circuit there may be a roll resilience switching valve. When the vehicle is traveling in a straight line, the valve may be held open to allow the second accumulators to communicate with the associated fluid circuits to provide additional roll resilience, thereby further improving ride comfort. When turning of the vehicle is detected, the roll resilience switching valve is closed to provide a desirable increase in roll stiffness during cornering. The detection of vehicle cornering may be performed in any known manner, using inputs for conditions such as steering rate of change, steering angle, lateral acceleration and vehicle speed. Any or all of these sensors and/or others not cited may be used.
The accumulators may be of the gas or mechanically sprung piston type or the diaphragm type and either or both can be beneficial in increasing the time to maintenance of the system by replenishing fluid lost from the system through leaks past rod seals and out of fittings. Any fluid loss should be minimal, therefore the effect on the operating pressure of the system may be negligible.
At least one of the accumulators in each fluid circuit may have a damper means to control the rate of fluid flow into and/or out of the accumulator. Due to the higher rate of fluid flow into and out of the accumulators in roll when compared to bounce (as discussed earlier), the effect of the accumulator dampers is greater in roll than in bounce giving a desirable high roll damping to bounce damping ratio. If the accumulators are not damped, the roll damping is determined by the bounce damping, as is the case when using conventional dampers.
Damping the accumulators can also have a detrimental effect to single wheel input harshness as single wheel inputs are also heavily damped by accumulator dampers. To increase comfort in straight line running, it can therefore be advantageous to provide a bypass passage around the accumulator damper valve to permit fluid to bypass the damper for at least one accumulator. The bypass passage includes a valve to open or close the passage. During turning, the valve is in the closed position and the accumulator damper valves are providing high roll damping. In straight line running, the valve is open to reduce the roll and single wheel input damping forces in the system.
The roll control system may have a pressure precharge to allow the accumulators to function and supply fluid in rebound motions of the wheels (where they fall away from the vehicle body). This precharge is preferably about 20 bar for the roll control system with the vehicle at standard uniaden ride height.
It may be preferable to use a wheel cylinder design with a rod protruding from one side of the piston through only one chamber. This allows for a simple and cheap cylinder design, but any system precharge pressure acting over the unequal effective piston areas in the first and second chambers produces a not cylinder force. This force may provide some support of the vehicle body although the proportion of vehicle load supported by the roll control system is usually very small and is similar to the degree of support provided by a conventional precharged damper cylinder assembly. The exact amount is determined by the cylinder rod and bore dimensions, system precharge pressure and cylinder to wheel hub lever ratio.
For example, in the case where the first chamber of each wheal cylinder is in compression as the wheels move upwardly with respect to the vehicle body, and the effective area of the piston on the first chamber side is larger than the effective area of said piston on the second chamber side, thereby providing a degree of support of the vehicle body.
If accumulators with a non-linear spring function (ie a hydropneumatic accumulator which has an increasing stiffness in compression and a decreasing stiffness in rebound) are used and the roll control system provides a degree of vehicle support (as outlined above), then as the vehicle rolls due to lateral acceleration, the total volume of fluid in the accumulators can decrease overall, increasing the fluid volume in the roll control system and causing an overall increase in vehicle height (known as xe2x80x9croll jackingxe2x80x9d). The degree of vehicle support provided by the roll control system influences the degree of roll jacking.
It may be desirable to produce the inverse of the roll jacking effect such that the average height of the vehicle is lowered during cornering. This effect can be produced in the case where the first chamber of each wheel cylinder is in compression as the wheels move upwardly with respect to the vehicle body, and the effective area of the piston on the second chamber side is larger than the effective area of said piston on the first chamber side, thereby providing a degree of additional load on the vehicle support means, tending to push the vehicle down towards the ground.
Preferably, a simpler arrangement may be used with the cheaper cylinder design which provides vehicle support (discussed above). The resilient means in the first accumulator may include one or more mechanical springs such that the spring rate in the compression direction from the normal static position is lower than the spring rate in the rebound direction from the normal static position, to thereby give the reverse effect of a conventional hydropneumatic accumulator and lower the average height of the vehicle during cornering. Additionally or alternatively, the rebound damping rate of the accumulators may be higher than the compression damping rate to provide a similar vehicle lowering effect and better response to steering inputs during initial cornering (turn-in). Indeed, only rebound damping may be provided for the accumulators, with a non-return valve allowing virtually unrestricted flow in the compression direction.
Ideally, the roll control system should not provide any vertical support of the vehicle. Therefore, in another, alternative preferred arrangement of the present invention, the effective piston areas in the first and second chambers of each cylinder may be similar, the roll control system thereby supporting substantially zero vehicle load. As the amount of vehicle load support provided by the roll control system is one of the main factors controlling the amount of roll jacking inherent in the system, using wheel cylinders with similar effective piston areas in the first and second chambers and which therefore do not provide any vehicle support provides the roll control system with zero roll jacking.
However, in some applications, the use of a cylinder having piston rods extending from both ends thereof can lead to packaging difficulties because of the need to provide clearance for the upwardly extending piston rod. Therefore, according to another preferred arrangement, a piston rod may extend from one side of the piston, the piston rod having as small a diameter as physically possible to minimise the vehicle support provided by the damping and roll control system. In another possible arrangement, a hollow piston rod may extend from one side of the piston, and an inner rod may be supported within the inner volume of the cylinder, the inner rod being at least partially accommodated within the hollow piston rod, the hollow piston rod moving together with the piston relative to the inner rod. This arrangement may be used to minimise the difference in area of the opposing piston faces to minimise the vehicle support provided by the damping and roll control system.
According to an alternative preferred embodiment, the hollow piston rod arrangement of the wheel cylinder may be adapted to also provide a vertical support function for the vehicle. The piston supported in the wheel cylinder may provide an upper and lower chamber. The inner rod when supported within the hollow piston rod defines a rod chamber. This rod chamber may be used as part of a fluid circuit of the roll control system. To this end, the area of the peripheral end of the inner rod may be at least substantially identical to the area of the piston facing the lower chamber. Alternatively, it can be preferable to use a larger lower chamber area than the rod chamber area to induce lowering of the vehicle in roll with increasing roll moment when hydropneumatic accumulators are used in the system.
The upper chamber may be sealed to provide a bounce chamber to provide resilient support for the vehicle. The rod chamber may be vented and, together with the lower chamber, form a respective part of a fluid circuit of the roll control chamber.
It should be noted that the roll moment distribution for the roll control system is determined by the ratio between the effective piston areas of the front wheel cylinders compared to the effective piston areas of the rear wheel cylinders. Ideally, in most applications, each wheel cylinder should have a constant ratio between the effective piston area on the first chamber side compared to the second chamber side.
One advantage of using cylinders where the piston rod is only provided extending from the one piston face is that the degree of support provided by the cylinders can be varied by varying the support height of the vehicle. As the vehicle is lowered the support provided by the roll control system increases leading to higher roll stiffness. This is an affect of having an increased volume of piston rod introduced into the roll control system.
The support means for at least one pair of laterally spaced wheel assemblies may include first support means which are independent for each wheel assembly, thereby contributing an additional roll stiffness to the suspension system. Both the vehicle support means and the roll control system can together provide the roll stiffness for the vehicle in this arrangement.
Additionally or alternatively, the support means for at least one pair of laterally spaced wheels may include second support means which are interconnected between each wheel thereby contributing substantially zero roll stiffness to the suspension system. This and other vehicle support arrangements that provide little to no roll support and combinations of support arrangements are described in the Applicants"" International Application No. PCT/AU97/00870 referred to previously. In such an arrangement, the damping and roll control system can provide substantially all of the roll control for the vehicle. Furthermore if the support means have substantially zero roll stiffness, the damping and roll control system can provide substantially all of the roll control for the vehicle. In this case, neither the support means or roll control system provides significant warp stiffness. This allows for substantially free warp motion of the vehicle wheel assemblies, improving comfort, reactions to single wheel inputs and providing substantially constant wheel loads (and therefore improved traction) in low speed or non-dynamic warp motions when traversing uneven terrain such as in off-road situations.
According to the present invention, the first and second fluid circuits are in fluid communication such that fluid may be transferred therebetween. To this end, at least one bridge passage may interconnect the first and second fluid circuits to provide for said fluid communication. The bridge passage may be provided by a bridge conduit. Alternatively, the bridge passage may be provided within a connector body to which the conduits of the first and second circuits are connected. At least one flow control valve may be provided for controlling the flow through the bridge passage.
One or more accumulators may optionally also be provided for the bridge passage. The flow control valve and accumulator may be provided on a said bridge conduit. According to another possible arrangement, the control valve and/or accumulator may be supported on the connector body. It is also possible for all the damper valves and accumulators previously referred to be located on a common said connector body to simplify the packaging of the system within a vehicle.
The flow control valve may be opened, for example when there is little demand on the roll control system when the vehicle is travelling on a straight road. When the flow control valve is opened, this leads to a xe2x80x9cshort-circuitingxe2x80x9d of the system such that the first and second chambers of each cylinder are allowed to communicate directly. This controlled interconnection of the first and second fluid circuits by the controlled opening of the flow control valve provide a number of operational advantages that lead to improved comfort for the passengers of the vehicle:
a) The damping and roll control system provides no roll stiffness, the only roll stiffness being provided by the vehicle support means.
b) The damping and roll control system no longer effects the roll split of the vehicle, the roll split only being provided by the vehicle support means. If the roll spilt provided by the vehicle support means is between approximately 40 and 60%, this (in combination with the low roll stiffness) acts to reduce vehicle motions leading to xe2x80x9chead tossxe2x80x9d.
c) As there is little resistance to the fluid flow between the chambers of each cylinder except for that provided by the wheel damper valve, the single wheel stiffness is reduced.
d) Because the accumulator damper valves are bypassed, they do not influence the damping function of the damping and roll control system, and the roll damping is the same as the bounce damping.
e) The single wheel damping is (for the same reason) the same as the bounce damping.
f) The bounce damping however remains unchanged when the flow control valve is opened.
The operation of the flow control valve may be controlled by an Electronic Control Unit on the basis of operational parameters such as the lateral acceleration speed and steering rate of the vehicle.
It is also possible for a plurality of bridge passages to be provided interconnecting the first and second fluid circuits. Each bridge passage may be provided with a said flow control valve.
It is also possible that the wheel cylinder include an integral flow control valve and/or damper valve therein. The piston of the wheel cylinder may include a flow control valve and(or damper valve controlling the flow of fluid between the first and second chambers.
The use of a plurality of bridge passages having flow control valves or wheel cylinders having built-in flow control valves facilitates fluid flow between the first and second chambers of the wheel cylinders. This can lead to a reduction in the inertia forces due to fluid flow through the system resulting in improved isolation of high frequency inputs and sharp edge inputs to the vehicle wheels. The effect of inertia forces within the roll control system will be subsequently described in more detail.
As the damping and roll control system can be switched to provide substantially zero roll stiffness, the use of zero roll stiffness support means for all wheels is not viable. However, zero roll stiffness support means may still be used in combination with independent support means providing some roll stiffness. Therefore, the support means for at least one pair of laterally spaced wheels may include first support means for supporting at least a portion of the load on the associated wheel assemblies, said first support means providing independent resilience for each respective wheel and thereby providing a roll stiffness.
Additionally, the support means for at least one pair of laterally spaced wheels may include second support means for supporting at least a portion of the load on the associated wheel assemblies, said second support means providing combined resilience for each associated wheel assembly and thereby providing substantially zero roll stiffness.
It is to be appreciated that the conduit size may be selected to provide a degree of the damping required by the damping and roll control system. Depending on the level of ride comfort required in an application, the conduit size may be selected based on a variety of factors such as fluid inertia, fluid friction due to viscosity through range of operating temperatures, etc.
The vehicle support means preferably provides most if not all the vertical support for the vehicle. The damping and roll control system however preferably provides little to no vertical support for the vehicle such that the operating fluid pressure within the damping and roll control system can therefore be relatively low when compared with active roll control systems and the Applicants"" earlier suspension systems. Theoretically, if the roll control system provides no vertical support for the vehicle, the operating pressure may be only atmospheric pressure, the system has no precharge pressure.
The damping and roll control system of the suspension system according to the present invention can therefore use low pressure components. The wheel cylinders can be constructed using standard vehicle damper and sealing technology. This leads to substantial manufacturing cost savings when compared to higher pressure systems. Also, comfort and NVH problems associated with higher pressure systems such as xe2x80x9cstictionxe2x80x9d between components are minimised in low pressure systems, the stiction levels being similar to that present in a conventional damper cylinder assembly.
Such a damping and roll control system can be installed in existing vehicle suspension systems, the dampers used in such systems being replaced or adapted for use as the wheel cylinders of the roll control system according to the present invention. The existing vehicle support means supporting the vehicle such as conventional steel or pneumatic springs can be retained. Alternatively, the vehicle support means may be replaced by support means that provide little to no roll support as described previously. This is possible because the damping and roll control system also provides a roll stiffness for the vehicle suspension system.
According to a further aspect of the present invention, there is provided a method of controlling the roll damping and roll stiffness of a damping and roll control system for a vehicle suspension system, the damping and roll control system including:
wheel cylinders respectively locatable at wheel assemblies of the vehicle, each wheel cylinder including an inner volume separated into first and second chambers by a piston supported within the wheel cylinder; and first and second fluid circuits respectively providing fluid communication between the wheel cylinders by fluid conduits, each said fluid circuit providing fluid communication between the first chambers of the wheel cylinders on one side of the vehicle and the second chambers of the wheel cylinders on the opposite side of the vehicle to thereby provide roll support decoupled from the warp mode of the vehicle suspension system by providing a roll stiffness about a level roll attitude whilst simultaneously providing substantially zero warp stiffness;
damper means for controlling the rate of fluid flow into and out of at least one chamber of each wheel cylinder.
the method including opening the selection means to provide fluid communication between the first and second fluid circuits when the lamping and roll system is required to provide a relatively low level of roll stiffness and roll damping; and
closing the selection means to prevent fluid communication between the first and second fluid circuits when the damping and roll system is required to provide a relatively high level of roll stiffness and roll damping.
The fluid flow may be bypassed from at least a substantial portion of the fluid conduits by opening the selection means when there is a single wheel input or two wheel parallel bump input to the damping and roll control system. The line damping and fluid inertia effects on the damping of the control system can therefore be minimised at such wheel inputs.
It is also envisaged that the entire fluid flow be bypassed from the fluid conduits at the predetermined wheel inputs. This can for example be achieved by providing a control valve within the wheel cylinder as hereinbefore described.
Damping means such as single and bi-directional damper valves may be provided through which the bypassed fluid flow passes, these damping means clearly controlling the damping of the control system during this operational mode.